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    <a href="https://codemirror.net"><h1>CodeMirror</h1><img id=logo src="logo.png"></a>

    <ul>
        <li><a href="../index.html">Home</a>
        <li><a href="manual.html">Manual</a>
        <li><a href="https://github.com/codemirror/codemirror">Code</a>
    </ul>
    <ul>
        <li><a href="#top">Introduction</a></li>
        <li><a href="#approach">General Approach</a></li>
        <li><a href="#input">Input</a></li>
        <li><a href="#selection">Selection</a></li>
        <li><a href="#update">Intelligent Updating</a></li>
        <li><a href="#parse">Parsing</a></li>
        <li><a href="#summary">What Gives?</a></li>
        <li><a href="#btree">Content Representation</a></li>
        <li><a href="#keymap">Key Maps</a></li>
    </ul>
</div>

<article>

    <h2 id=top>(Re-) Implementing A Syntax-Highlighting Editor in JavaScript</h2>

    <p id="intro" style="font-size: 85%">
        <strong>Topic:</strong> JavaScript, code editor implementation<br>
        <strong>Author:</strong> Marijn Haverbeke<br>
        <strong>Date:</strong> March 2nd 2011 (updated November 13th 2011)
    </p>

    <p style="padding: 0 3em 0 2em"><strong>Caution</strong>: this text was written briefly after
        version 2 was initially written. It no longer (even including the
        update at the bottom) fully represents the current implementation. I'm
        leaving it here as a historic document. For more up-to-date
        information, look at the entries
        tagged <a href="http://marijnhaverbeke.nl/blog/#cm-internals">cm-internals</a>
        on my blog.</p>

    <p>This is a followup to
        my <a href="https://codemirror.net/story.html">Brutal Odyssey to the
            Dark Side of the DOM Tree</a> story. That one describes the
        mind-bending process of implementing (what would become) CodeMirror 1.
        This one describes the internals of CodeMirror 2, a complete rewrite
        and rethink of the old code base. I wanted to give this piece another
        Hunter Thompson copycat subtitle, but somehow that would be out of
        place—the process this time around was one of straightforward
        engineering, requiring no serious mind-bending whatsoever.</p>

    <p>So, what is wrong with CodeMirror 1? I'd estimate, by mailing list
        activity and general search-engine presence, that it has been
        integrated into about a thousand systems by now. The most prominent
        one, since a few weeks,
        being <a href="http://googlecode.blogspot.com/2011/01/make-quick-fixes-quicker-on-google.html">Google
            code's project hosting</a>. It works, and it's being used widely.</p>

    <p>Still, I did not start replacing it because I was bored. CodeMirror
        1 was heavily reliant on <code>designMode</code>
        or <code>contentEditable</code> (depending on the browser). Neither of
        these are well specified (HTML5 tries
        to <a href="http://www.w3.org/TR/html5/editing.html#contenteditable">specify</a>
        their basics), and, more importantly, they tend to be one of the more
        obscure and buggy areas of browser functionality—CodeMirror, by using
        this functionality in a non-typical way, was constantly running up
        against browser bugs. WebKit wouldn't show an empty line at the end of
        the document, and in some releases would suddenly get unbearably slow.
        Firefox would show the cursor in the wrong place. Internet Explorer
        would insist on linkifying everything that looked like a URL or email
        address, a behaviour that can't be turned off. Some bugs I managed to
        work around (which was often a frustrating, painful process), others,
        such as the Firefox cursor placement, I gave up on, and had to tell
        user after user that they were known problems, but not something I
        could help.</p>

    <p>Also, there is the fact that <code>designMode</code> (which seemed
        to be less buggy than <code>contentEditable</code> in Webkit and
        Firefox, and was thus used by CodeMirror 1 in those browsers) requires
        a frame. Frames are another tricky area. It takes some effort to
        prevent getting tripped up by domain restrictions, they don't
        initialize synchronously, behave strangely in response to the back
        button, and, on several browsers, can't be moved around the DOM
        without having them re-initialize. They did provide a very nice way to
        namespace the library, though—CodeMirror 1 could freely pollute the
        namespace inside the frame.</p>

    <p>Finally, working with an editable document means working with
        selection in arbitrary DOM structures. Internet Explorer (8 and
        before) has an utterly different (and awkward) selection API than all
        of the other browsers, and even among the different implementations of
        <code>document.selection</code>, details about how exactly a selection
        is represented vary quite a bit. Add to that the fact that Opera's
        selection support tended to be very buggy until recently, and you can
        imagine why CodeMirror 1 contains 700 lines of selection-handling
        code.</p>

    <p>And that brings us to the main issue with the CodeMirror 1
        code base: The proportion of browser-bug-workarounds to real
        application code was getting dangerously high. By building on top of a
        few dodgy features, I put the system in a vulnerable position—any
        incompatibility and bugginess in these features, I had to paper over
        with my own code. Not only did I have to do some serious stunt-work to
        get it to work on older browsers (as detailed in the
        previous <a href="https://codemirror.net/story.html">story</a>), things
        also kept breaking in newly released versions, requiring me to come up
        with <em>new</em> scary hacks in order to keep up. This was starting
        to lose its appeal.</p>

    <section id=approach>
        <h2>General Approach</h2>

        <p>What CodeMirror 2 does is try to sidestep most of the hairy hacks
            that came up in version 1. I owe a lot to the
            <a href="http://ace.ajax.org">ACE</a> editor for inspiration on how to
            approach this.</p>

        <p>I absolutely did not want to be completely reliant on key events to
            generate my input. Every JavaScript programmer knows that key event
            information is horrible and incomplete. Some people (most awesomely
            Mihai Bazon with <a href="http://ymacs.org">Ymacs</a>) have been able
            to build more or less functioning editors by directly reading key
            events, but it takes a lot of work (the kind of never-ending, fragile
            work I described earlier), and will never be able to properly support
            things like multi-keystoke international character
            input. <a class="update" href="#keymap">[see below for caveat]</a></p>

        <p>So what I do is focus a hidden textarea, and let the browser
            believe that the user is typing into that. What we show to the user is
            a DOM structure we built to represent his document. If this is updated
            quickly enough, and shows some kind of believable cursor, it feels
            like a real text-input control.</p>

        <p>Another big win is that this DOM representation does not have to
            span the whole document. Some CodeMirror 1 users insisted that they
            needed to put a 30 thousand line XML document into CodeMirror. Putting
            all that into the DOM takes a while, especially since, for some
            reason, an editable DOM tree is slower than a normal one on most
            browsers. If we have full control over what we show, we must only
            ensure that the visible part of the document has been added, and can
            do the rest only when needed. (Fortunately, the <code>onscroll</code>
            event works almost the same on all browsers, and lends itself well to
            displaying things only as they are scrolled into view.)</p>
    </section>
    <section id="input">
        <h2>Input</h2>

        <p>ACE uses its hidden textarea only as a text input shim, and does
            all cursor movement and things like text deletion itself by directly
            handling key events. CodeMirror's way is to let the browser do its
            thing as much as possible, and not, for example, define its own set of
            key bindings. One way to do this would have been to have the whole
            document inside the hidden textarea, and after each key event update
            the display DOM to reflect what's in that textarea.</p>

        <p>That'd be simple, but it is not realistic. For even medium-sized
            document the editor would be constantly munging huge strings, and get
            terribly slow. What CodeMirror 2 does is put the current selection,
            along with an extra line on the top and on the bottom, into the
            textarea.</p>

        <p>This means that the arrow keys (and their ctrl-variations), home,
            end, etcetera, do not have to be handled specially. We just read the
            cursor position in the textarea, and update our cursor to match it.
            Also, copy and paste work pretty much for free, and people get their
            native key bindings, without any special work on my part. For example,
            I have emacs key bindings configured for Chrome and Firefox. There is
            no way for a script to detect this. <a class="update"
                                                   href="#keymap">[no longer the case]</a></p>

        <p>Of course, since only a small part of the document sits in the
            textarea, keys like page up and ctrl-end won't do the right thing.
            CodeMirror is catching those events and handling them itself.</p>
    </section>
    <section id="selection">
        <h2>Selection</h2>

        <p>Getting and setting the selection range of a textarea in modern
            browsers is trivial—you just use the <code>selectionStart</code>
            and <code>selectionEnd</code> properties. On IE you have to do some
            insane stuff with temporary ranges and compensating for the fact that
            moving the selection by a 'character' will treat \r\n as a single
            character, but even there it is possible to build functions that
            reliably set and get the selection range.</p>

        <p>But consider this typical case: When I'm somewhere in my document,
            press shift, and press the up arrow, something gets selected. Then, if
            I, still holding shift, press the up arrow again, the top of my
            selection is adjusted. The selection remembers where its <em>head</em>
            and its <em>anchor</em> are, and moves the head when we shift-move.
            This is a generally accepted property of selections, and done right by
            every editing component built in the past twenty years.</p>

        <p>But not something that the browser selection APIs expose.</p>

        <p>Great. So when someone creates an 'upside-down' selection, the next
            time CodeMirror has to update the textarea, it'll re-create the
            selection as an 'upside-up' selection, with the anchor at the top, and
            the next cursor motion will behave in an unexpected way—our second
            up-arrow press in the example above will not do anything, since it is
            interpreted in exactly the same way as the first.</p>

        <p>No problem. We'll just, ehm, detect that the selection is
            upside-down (you can tell by the way it was created), and then, when
            an upside-down selection is present, and a cursor-moving key is
            pressed in combination with shift, we quickly collapse the selection
            in the textarea to its start, allow the key to take effect, and then
            combine its new head with its old anchor to get the <em>real</em>
            selection.</p>

        <p>In short, scary hacks could not be avoided entirely in CodeMirror
            2.</p>

        <p>And, the observant reader might ask, how do you even know that a
            key combo is a cursor-moving combo, if you claim you support any
            native key bindings? Well, we don't, but we can learn. The editor
            keeps a set known cursor-movement combos (initialized to the
            predictable defaults), and updates this set when it observes that
            pressing a certain key had (only) the effect of moving the cursor.
            This, of course, doesn't work if the first time the key is used was
            for extending an inverted selection, but it works most of the
            time.</p>
    </section>
    <section id="update">
        <h2>Intelligent Updating</h2>

        <p>One thing that always comes up when you have a complicated internal
            state that's reflected in some user-visible external representation
            (in this case, the displayed code and the textarea's content) is
            keeping the two in sync. The naive way is to just update the display
            every time you change your state, but this is not only error prone
            (you'll forget), it also easily leads to duplicate work on big,
            composite operations. Then you start passing around flags indicating
            whether the display should be updated in an attempt to be efficient
            again and, well, at that point you might as well give up completely.</p>

        <p>I did go down that road, but then switched to a much simpler model:
            simply keep track of all the things that have been changed during an
            action, and then, only at the end, use this information to update the
            user-visible display.</p>

        <p>CodeMirror uses a concept of <em>operations</em>, which start by
            calling a specific set-up function that clears the state and end by
            calling another function that reads this state and does the required
            updating. Most event handlers, and all the user-visible methods that
            change state are wrapped like this. There's a method
            called <code>operation</code> that accepts a function, and returns
            another function that wraps the given function as an operation.</p>

        <p>It's trivial to extend this (as CodeMirror does) to detect nesting,
            and, when an operation is started inside an operation, simply
            increment the nesting count, and only do the updating when this count
            reaches zero again.</p>

        <p>If we have a set of changed ranges and know the currently shown
            range, we can (with some awkward code to deal with the fact that
            changes can add and remove lines, so we're dealing with a changing
            coordinate system) construct a map of the ranges that were left
            intact. We can then compare this map with the part of the document
            that's currently visible (based on scroll offset and editor height) to
            determine whether something needs to be updated.</p>

        <p>CodeMirror uses two update algorithms—a full refresh, where it just
            discards the whole part of the DOM that contains the edited text and
            rebuilds it, and a patch algorithm, where it uses the information
            about changed and intact ranges to update only the out-of-date parts
            of the DOM. When more than 30 percent (which is the current heuristic,
            might change) of the lines need to be updated, the full refresh is
            chosen (since it's faster to do than painstakingly finding and
            updating all the changed lines), in the other case it does the
            patching (so that, if you scroll a line or select another character,
            the whole screen doesn't have to be
            re-rendered). <span class="update">[the full-refresh
algorithm was dropped, it wasn't really faster than the patching
one]</span></p>

        <p>All updating uses <code>innerHTML</code> rather than direct DOM
            manipulation, since that still seems to be by far the fastest way to
            build documents. There's a per-line function that combines the
            highlighting, <a href="manual.html#markText">marking</a>, and
            selection info for that line into a snippet of HTML. The patch updater
            uses this to reset individual lines, the refresh updater builds an
            HTML chunk for the whole visible document at once, and then uses a
            single <code>innerHTML</code> update to do the refresh.</p>
    </section>
    <section id="parse">
        <h2>Parsers can be Simple</h2>

        <p>When I wrote CodeMirror 1, I
            thought <a href="https://codemirror.net/story.html#parser">interruptable
                parsers</a> were a hugely scary and complicated thing, and I used a
            bunch of heavyweight abstractions to keep this supposed complexity
            under control: parsers
            were <a href="http://bob.pythonmac.org/archives/2005/07/06/iteration-in-javascript/">iterators</a>
            that consumed input from another iterator, and used funny
            closure-resetting tricks to copy and resume themselves.</p>

        <p>This made for a rather nice system, in that parsers formed strictly
            separate modules, and could be composed in predictable ways.
            Unfortunately, it was quite slow (stacking three or four iterators on
            top of each other), and extremely intimidating to people not used to a
            functional programming style.</p>

        <p>With a few small changes, however, we can keep all those
            advantages, but simplify the API and make the whole thing less
            indirect and inefficient. CodeMirror
            2's <a href="manual.html#modeapi">mode API</a> uses explicit state
            objects, and makes the parser/tokenizer a function that simply takes a
            state and a character stream abstraction, advances the stream one
            token, and returns the way the token should be styled. This state may
            be copied, optionally in a mode-defined way, in order to be able to
            continue a parse at a given point. Even someone who's never touched a
            lambda in his life can understand this approach. Additionally, far
            fewer objects are allocated in the course of parsing now.</p>

        <p>The biggest speedup comes from the fact that the parsing no longer
            has to touch the DOM though. In CodeMirror 1, on an older browser, you
            could <em>see</em> the parser work its way through the document,
            managing some twenty lines in each 50-millisecond time slice it got. It
            was reading its input from the DOM, and updating the DOM as it went
            along, which any experienced JavaScript programmer will immediately
            spot as a recipe for slowness. In CodeMirror 2, the parser usually
            finishes the whole document in a single 100-millisecond time slice—it
            manages some 1500 lines during that time on Chrome. All it has to do
            is munge strings, so there is no real reason for it to be slow
            anymore.</p>
    </section>
    <section id="summary">
        <h2>What Gives?</h2>

        <p>Given all this, what can you expect from CodeMirror 2?</p>

        <ul>

            <li><strong>Small.</strong> the base library is
                some <span class="update">45k</span> when minified
                now, <span class="update">17k</span> when gzipped. It's smaller than
                its own logo.
            </li>

            <li><strong>Lightweight.</strong> CodeMirror 2 initializes very
                quickly, and does almost no work when it is not focused. This means
                you can treat it almost like a textarea, have multiple instances on a
                page without trouble.
            </li>

            <li><strong>Huge document support.</strong> Since highlighting is
                really fast, and no DOM structure is being built for non-visible
                content, you don't have to worry about locking up your browser when a
                user enters a megabyte-sized document.
            </li>

            <li><strong>Extended API.</strong> Some things kept coming up in the
                mailing list, such as marking pieces of text or lines, which were
                extremely hard to do with CodeMirror 1. The new version has proper
                support for these built in.
            </li>

            <li><strong>Tab support.</strong> Tabs inside editable documents were,
                for some reason, a no-go. At least six different people announced they
                were going to add tab support to CodeMirror 1, none survived (I mean,
                none delivered a working version). CodeMirror 2 no longer removes tabs
                from your document.
            </li>

            <li><strong>Sane styling.</strong> <code>iframe</code> nodes aren't
                really known for respecting document flow. Now that an editor instance
                is a plain <code>div</code> element, it is much easier to size it to
                fit the surrounding elements. You don't even have to make it scroll if
                you do not <a href="../demo/resize.html">want to</a>.
            </li>

        </ul>

        <p>On the downside, a CodeMirror 2 instance is <em>not</em> a native
            editable component. Though it does its best to emulate such a
            component as much as possible, there is functionality that browsers
            just do not allow us to hook into. Doing select-all from the context
            menu, for example, is not currently detected by CodeMirror.</p>

        <p id="changes" style="margin-top: 2em;"><span style="font-weight:
bold">[Updates from November 13th 2011]</span> Recently, I've made
            some changes to the codebase that cause some of the text above to no
            longer be current. I've left the text intact, but added markers at the
            passages that are now inaccurate. The new situation is described
            below.</p>
    </section>
    <section id="btree">
        <h2>Content Representation</h2>

        <p>The original implementation of CodeMirror 2 represented the
            document as a flat array of line objects. This worked well—splicing
            arrays will require the part of the array after the splice to be
            moved, but this is basically just a simple <code>memmove</code> of a
            bunch of pointers, so it is cheap even for huge documents.</p>

        <p>However, I recently added line wrapping and code folding (line
            collapsing, basically). Once lines start taking up a non-constant
            amount of vertical space, looking up a line by vertical position
            (which is needed when someone clicks the document, and to determine
            the visible part of the document during scrolling) can only be done
            with a linear scan through the whole array, summing up line heights as
            you go. Seeing how I've been going out of my way to make big documents
            fast, this is not acceptable.</p>

        <p>The new representation is based on a B-tree. The leaves of the tree
            contain arrays of line objects, with a fixed minimum and maximum size,
            and the non-leaf nodes simply hold arrays of child nodes. Each node
            stores both the amount of lines that live below them and the vertical
            space taken up by these lines. This allows the tree to be indexed both
            by line number and by vertical position, and all access has
            logarithmic complexity in relation to the document size.</p>

        <p>I gave line objects and tree nodes parent pointers, to the node
            above them. When a line has to update its height, it can simply walk
            these pointers to the top of the tree, adding or subtracting the
            difference in height from each node it encounters. The parent pointers
            also make it cheaper (in complexity terms, the difference is probably
            tiny in normal-sized documents) to find the current line number when
            given a line object. In the old approach, the whole document array had
            to be searched. Now, we can just walk up the tree and count the sizes
            of the nodes coming before us at each level.</p>

        <p>I chose B-trees, not regular binary trees, mostly because they
            allow for very fast bulk insertions and deletions. When there is a big
            change to a document, it typically involves adding, deleting, or
            replacing a chunk of subsequent lines. In a regular balanced tree, all
            these inserts or deletes would have to be done separately, which could
            be really expensive. In a B-tree, to insert a chunk, you just walk
            down the tree once to find where it should go, insert them all in one
            shot, and then break up the node if needed. This breaking up might
            involve breaking up nodes further up, but only requires a single pass
            back up the tree. For deletion, I'm somewhat lax in keeping things
            balanced—I just collapse nodes into a leaf when their child count goes
            below a given number. This means that there are some weird editing
            patterns that may result in a seriously unbalanced tree, but even such
            an unbalanced tree will perform well, unless you spend a day making
            strangely repeating edits to a really big document.</p>
    </section>
    <section id="keymap">
        <h2>Keymaps</h2>

        <p><a href="#approach">Above</a>, I claimed that directly catching key
            events for things like cursor movement is impractical because it
            requires some browser-specific kludges. I then proceeded to explain
            some awful <a href="#selection">hacks</a> that were needed to make it
            possible for the selection changes to be detected through the
            textarea. In fact, the second hack is about as bad as the first.</p>

        <p>On top of that, in the presence of user-configurable tab sizes and
            collapsed and wrapped lines, lining up cursor movement in the textarea
            with what's visible on the screen becomes a nightmare. Thus, I've
            decided to move to a model where the textarea's selection is no longer
            depended on.</p>

        <p>So I moved to a model where all cursor movement is handled by my
            own code. This adds support for a goal column, proper interaction of
            cursor movement with collapsed lines, and makes it possible for
            vertical movement to move through wrapped lines properly, instead of
            just treating them like non-wrapped lines.</p>

        <p>The key event handlers now translate the key event into a string,
            something like <code>Ctrl-Home</code> or <code>Shift-Cmd-R</code>, and
            use that string to look up an action to perform. To make keybinding
            customizable, this lookup goes through
            a <a href="manual.html#option_keyMap">table</a>, using a scheme that
            allows such tables to be chained together (for example, the default
            Mac bindings fall through to a table named 'emacsy', which defines
            basic Emacs-style bindings like <code>Ctrl-F</code>, and which is also
            used by the custom Emacs bindings).</p>

        <p>A new
            option <a href="manual.html#option_extraKeys"><code>extraKeys</code></a>
            allows ad-hoc keybindings to be defined in a much nicer way than what
            was possible with the
            old <a href="manual.html#option_onKeyEvent"><code>onKeyEvent</code></a>
            callback. You simply provide an object mapping key identifiers to
            functions, instead of painstakingly looking at raw key events.</p>

        <p>Built-in commands map to strings, rather than functions, for
            example <code>"goLineUp"</code> is the default action bound to the up
            arrow key. This allows new keymaps to refer to them without
            duplicating any code. New commands can be defined by assigning to
            the <code>CodeMirror.commands</code> object, which maps such commands
            to functions.</p>

        <p>The hidden textarea now only holds the current selection, with no
            extra characters around it. This has a nice advantage: polling for
            input becomes much, much faster. If there's a big selection, this text
            does not have to be read from the textarea every time—when we poll,
            just noticing that something is still selected is enough to tell us
            that no new text was typed.</p>

        <p>The reason that cheap polling is important is that many browsers do
            not fire useful events on IME (input method engine) input, which is
            the thing where people inputting a language like Japanese or Chinese
            use multiple keystrokes to create a character or sequence of
            characters. Most modern browsers fire <code>input</code> when the
            composing is finished, but many don't fire anything when the character
            is updated <em>during</em> composition. So we poll, whenever the
            editor is focused, to provide immediate updates of the display.</p>

    </section>
</article>
